primitive can be used to per-feature postprocess

[lafflan]

before this, set primitive selected in the per-feature postprocess had no effect, because they can't get the primitive's pickIds.
add a readonly property to insure primitive can be used to per-feature postprocess

[cesium-concierge]

Thank you so much for the pull request @sweerwen! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

The Pull Request Guidelines is a handy reference for making sure your PR gets accepted quickly, so make sure to skim that.

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[lafflan]

#7984 (comment)

[cesium-concierge]

Thanks again for your contribution @sweerwen!

No one has commented on this pull request in 30 days. Maintainers, can you review, merge or close to keep things tidy?

I'm going to re-bump this in 30 days. If you'd like me to stop, just comment with @cesium-concierge stop. If you want me to start again, just delete the comment.